Acute lung injury (ALI) is a common cause of respiratory failure in critically ill adults, with an incidence of nearly 200,000 cases/year in the US alone and a mortality of 30-40%.1 ALI frequently follows major trauma, which is itself the leading cause of mortality nationally between the ages of 1 and 44;2 further, the development of ALI following trauma increases mortality by 3-fold.3 We recently discovered that both active smoking and moderate to heavy secondhand smoke exposure are associated with a nearly 3-fold increase in the odds of developing ALI after severe blunt trauma, independent of alcohol abuse. In the research proposed here, we will test the central hypothesis that both active smoking and secondhand smoke exposure prior to trauma predispose patients to develop ALI via injury to the lung epithelium and endothelium and enhanced susceptibility to infection. We will use the infrastructure of our established, ongoing prospective cohort of severely injured blunt trauma patients at San Francisco General Hospital to collect the necessary clinical data and biologic specimens in 600 new patients to study three specific aims. For all aims, both active and passive cigarette smoke exposure will be rigorously quantified by well-validated biomarkers: specifically, plasma cotinine and urine total 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). In Aim 1, we will test the association between low-level secondhand smoke exposure and susceptibility to ALI after severe blunt trauma, since our prior studies were not large enough to address this level of exposure, which has major relevance to public health since it remains common in the US and internationally. In Aim 2, we will determine the extent to which active smoking and/or secondhand smoke exposure prime patients to develop ALI via endothelial and lung epithelial injury. Endothelial and lung epithelial injury will be quantified by measurement of specific, previously studied and validated protein biomarkers in plasma and bronchoalveolar lavage. In Aim 3, we will determine the extent to which active smoking and/or secondhand smoke exposure prime patients to develop ALI via enhanced susceptibility to infection, as reflected by changes in the lung microbiome. Our research group is well-qualified to conduct this research by virtue of our expertise in enrolling cohorts of severely injured trauma patients, measuring biomarkers of cigarette smoke exposure and of lung injury, characterizing the microbiome, and our history of successful collaboration. This project will significantly advance the field of ALI research by providing insight into how cigarette smoke primes patients to develop ALI, laying the groundwork for targeted and/or preventative therapies. In addition, it will likely have important public health implications regarding the regulation of secondhand smoke exposure. Our approach is especially innovative because of its focus on chronic environmental influences on the etiology of ALI, which have not been well studied; the use of biomarkers to measure exposure in critically ill subjects; its potential implications for prevention of ALI, a major priority of a recent NHLBI Working Group; and the insights it will provide into the effects of cigarette smoke exposure on the lung microbiome.